The gaseous envelope surrounding the Earth, divided into five layers - the troposphere , which contains half of the earth's atmosphere,   the stratosphere , the mesosphere, the thermosphere and the exosphere, which is the outer boundary of the atmosphere. The dry atmosphere consists almost entirely of nitrogen (78.1% volumetric mixing ratio) and oxygen (20.9% volumetric mixing ratio), along with many trace gases such as argon (0.93% volumetric mixing ratio), helium, and radiatively active greenhouse gases gases (GHG) , such as carbon dioxide (CO ₂ ) (volumetric mixing ratio 0.04 %) a ozone (O ₃ ) . In addition, the atmosphere contains water vapor GHG (H ₂O), the amounts of which are highly variable, but typically around 1 % volume mixing ratio. The atmosphere also contains clouds and aerosols .